[RFC][PATCHv2 03/12] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag

[Sergey Senozhatsky]

By setting or clearing V4L2_FLAG_MEMORY_NON_CONSISTENT flag
user-space should be able to set or clear queue's NON_CONSISTENT
->dma_attrs. Queue's ->dma_attrs are passed to the underlying
allocator in __vb2_buf_mem_alloc(), so thus user-space is able
to request vb2 buffer's memory to be either consistent (coherent)
or non-consistent.

Change-Id: Ib333081c482e23c9a89386078293e19c3fd59076
Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 Documentation/media/uapi/v4l/buffer.rst | 27 +++++++++++++++++++++++++
 include/uapi/linux/videodev2.h          |  2 ++
 2 files changed, 29 insertions(+)

diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
index 9149b57728e5..af007daf0591 100644
--- a/Documentation/media/uapi/v4l/buffer.rst
+++ b/Documentation/media/uapi/v4l/buffer.rst
@@ -705,6 +705,33 @@ Buffer Flags
 
 .. c:type:: v4l2_memory
 
+Memory Consistency Flags
+========================
+
+.. tabularcolumns:: |p{7.0cm}|p{2.2cm}|p{8.3cm}|
+
+.. cssclass:: longtable
+
+.. flat-table::
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       3 1 4
+
+    * .. _`V4L2_FLAG_MEMORY_NON_CONSISTENT`:
+
+      - ``V4L2_FLAG_MEMORY_NON_CONSISTENT``
+      - 0x00000001
+      - vb2 buffer is allocated either in consistent (it will be automatically
+	coherent between CPU and bus) or non-consistent memory. The latter
+	can provide performance gains, for instance CPU cache sync/flush
+	operations can be avoided if the buffer is accesed by the corresponding
+	device only and CPU does not read/write to/from that buffer. However,
+	this requires extra care from the driver -- it must guarantee memory
+	consistency by issuing cache flush/sync when consistency is needed.
+	If this flag is set V4L2 will attempt to allocate vb2 buffer in
+	non-consistent memory. This flag is ignored if queue does not report
+        :ret:`V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS` capability.
+
 enum v4l2_memory
 ================
 
diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
index 5f9357dcb060..72efc1c544cd 100644
--- a/include/uapi/linux/videodev2.h
+++ b/include/uapi/linux/videodev2.h
@@ -189,6 +189,8 @@ enum v4l2_memory {
 	V4L2_MEMORY_DMABUF           = 4,
 };
 
+#define V4L2_FLAG_MEMORY_NON_CONSISTENT		(1 << 0)
+
 /* see also http://vektor.theorem.ca/graphics/ycbcr/ */
 enum v4l2_colorspace {
 	/*
-- 
2.25.0.341.g760bfbb309-goog